Geothermal system using CFD modeling

Description

Introduction:

Renewable energy is a critical issue facing society today. Geothermal energy is a renewable resource that harnesses the Earth’s internal heat. Geothermal systems utilize wells to access this heat:  cold water or other fluids are injected (inlet) deep underground, where the Earth’s increasing temperature gradient heats them. The heated fluid is then extracted (Outlet).

This project employs Computational Fluid Dynamics (CFD) software to simulate a geothermal system and analyze fluid behavior. The geothermal system geometry was created in SpaceClaim, the mesh was generated in ANSYS Meshing, and the governing equations (continuity, momentum, turbulence, and energy) were solved using ANSYS Fluent.

Geometry:

The geometry of geothermal was drawn at SpaceClaim. The simulation domain measures 2000 m × 2000 m × 6000 m. Due to geometrical symmetry across the mid-y-z plane, the model only simulates the lower half. A 500 m × 500 m × 500 m fractured reservoir is positioned 4000 m below the surface, centered in the x-y plane. The injection and production wells, each with a 0.2 m diameter, are 400 m apart and symmetrically located about the y-z plane.

Mesh:

Following the creation of the geothermal system geometry using SpaceClaim (or similar CAD software), the model was imported into ANSYS Meshing for mesh generation. A conformal, structured mesh containing approximately 7.4 million elements was created. The use of a structured mesh allows for improved accuracy and efficiency in the subsequent CFD simulations. The choice of conformal meshing ensures that the mesh conforms to the complex geometry of the geothermal system, particularly around the wells and the fractured reservoir.

Setup:

This project utilized a steady-state, pressure-based solver. The k-ε standard turbulence model and an energy equation were included. The fractured reservoir is a porous zone with porosity of 0.047. Water was injected into the system at a velocity of 2.8 m/s and a temperature of 290 K.  A pressure outlet boundary condition was applied at the Outlet. A constant geothermal gradient of 3 K per 100 m was implemented, representing the increase in temperature with depth. The SIMPLE algorithm was employed for pressure-velocity coupling

Results:

The primary simulation result is the inlet and outlet water temperatures, presented in Table 1. With an inlet temperature of 290 K, the outlet temperature reached 371.5 K (98.5 °K), demonstrating effective heat transfer from the geothermal reservoir.

Table 1: Temperature of Inlet and Outlet